Pension Plans, Retirement Benefits and Savings Plans (details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pension Plans [Member]
Defined benefit plan disclosure
Service cost	$ 30	$ 29	$ 90	$ 88
Interest cost on benefit obligation	35	30	96	91
Expected return on plan assets	(60)	(57)	(180)	(172)
Net periodic benefit cost, settlement				1
Net periodic benefit cost, amortization of unrecognized prior service benefit	(1)		(1)
Net periodic benefit cost, amortization of unrecognized net actuarial loss	29	17	66	50
Total non-service cost (benefit)	3	(10)	(19)	(30)
Net periodic benefit cost	33	19	71	58
Pension Plans [Member] | Claims and claim adjustment expenses [Member]
Defined benefit plan disclosure
Service cost	13	12	37	36
Total non-service cost (benefit)	1	(4)	(8)	(13)
Pension Plans [Member] | General and administrative expenses [Member]
Defined benefit plan disclosure
Service cost	17	17	53	52
Total non-service cost (benefit)	2	(6)	(11)	(17)
Postretirement Benefit Plans [Member]
Defined benefit plan disclosure
Interest cost on benefit obligation	2	2	5	6
Net periodic benefit cost, amortization of unrecognized prior service benefit	(1)	(1)	(3)	(3)
Total non-service cost (benefit)	1	1	2	3
Net periodic benefit cost	1	1	2	3
Postretirement Benefit Plans [Member] | Claims and claim adjustment expenses [Member]
Defined benefit plan disclosure
Total non-service cost (benefit)	$ 1		1	1
Postretirement Benefit Plans [Member] | General and administrative expenses [Member]
Defined benefit plan disclosure
Total non-service cost (benefit)		$ 1	$ 1	$ 2